This invention relates generally to a method of an apparatus for tomographically examining an object by radiation such as X-rays or gamma rays for obtaining two or three dimensional information of the object.
Attempts have been made in the past to obtain two or three dimensional information about an object for both diagnostic and inspection purposes. In this connection it will be appreciated that when patients are involved, it is of the utmost importance to keep the exposure to X-rays and gamma rays at an absolute minimum.
Thus, the U.S. Pat. No. to Webb 3,580,997 employs a television camera to convert a radiographic image produced by a test object into video signals. This is used as an on line inspection system incorporating a go, no-go system whereby a test object or sample is compared with a standard. The patent to Siedband 3,582,651 discloses an image storage and display system intended for use with X-ray apparatus. The system has various modes such as a stop action mode, an intermittent slide mode and the like. In the diagnostic area, the patent to Corrigan, Jr. 3,679,823 reveals among other things an image intensifier used to intensify X-ray images obtained from a patient. The output of the image intensifirer is then recorded by a television camera. Much effort was expanded to improve X-ray imaging techniques prior to the work of Hounsfield who is credited with the prime mover in computer assisted tomography.
The Hounsfield patent 3,778,614, assigned to EMI, Limited, England, discloses a now commercial apparatus for examining a body such as the head of a patient with X-rays. This system of tomography has also been described in a series of three articles which appear in the Dec. 1973 issue of the BRITISH JOURNAL OF RADIOLOGY, Volume 46. The first article by Hounsfield appears on pgs. 1016 through 1022. A second paper by Ambrose appears on pgs. 1023 through 1047 and deals with the clinical application of the apparatus. A third paper by Perry and Bridges on pgs. 1048 through 1051 relates to radiation dose considerations.
The Hounsfield apparatus as described in that U.S. patent and elsewhere utilizes a pencil beam having a small cross section. He proposes to rotate the source of radiation to scan the head or object linearly to take 160 readings of X-ray transmissions per scan at the end of scan one. His system is rotated 1.degree. for scan 2, then 1.degree. for scan 3, etc. through 180.degree. for a total of 28,800 readings by the detector.
The recorder consists of only one scintillation crystal and this is somewhat inefficient and hence the exposure is relatively long. Also, his detector is basically a point detector rather than an area detector. The device requires a plurality of rays to achieve the desired imaging wherein the various scans are parallel to each other whereupon the angle is changed to take each succeeding set of readings. While the system is said to be one hundred times more sensitive than conventional X-ray systems, it still requires one hundred sixty readings for each transmission and a corresponding amount of time. In any case, the exposure to the patient is between 1 and 2.5 R (roetgens).
Since the 3,778,614 patent in 1971 Hounsfield has been granted various patents, including 4,052,618 which is a division of patent 3,944,833, which itself is a division of patent 3,866,047, this application being a continuation-in-part of patent 3,778,614 which is a continuation of Ser. No. 861,538, filed in 1969.
In most tomographic instruments the X-ray source moves in one direction and the photographic film simultaneously moves in the opposite direction. The patient lies in between. If the patient's body is regarded as a series of planes parallel to the film, there is only one plane whose projected image remains stationary with respect to the film as the film moves. Hence, that plane remains sharply focused on the film and the others are blurred. To greatly simplify the procedure, the X-rays can be directed so that they pass through the patient in parallel rays as the patient (or the apparatus) is rotated in steps around a single axis. If an image were made to each step, structures in the patient's body lying in a plane perpendicular to the axis of rotation would be recorded as a single one-dimensional line. By measuring the X-ray density along that line on each image, one isolates the information from the desired plane. Thus, one can reconstruct the single two-dimensional plane and then merely stack a sequence of such planes to get a full three-dimensional picture.
The differences and the advantages of the instant system over those of Hounsfield as seen in his patent 3,778,614 and others assigned to E.P.M.I. are many in number. The system of Hounsfield, as especially espoused in his patent 3,965,357, employs a plurality of lateral scans at a succession of orbital positions so that a plane section of the body is traversed by a set of parallel beams at each of the oriented positions. The system of the instant invention does not employ or require a plurality of rotations. Here only a single rotation is to be carried out. The key to this invention is the use of the charge couple device (CCD).
In view of the program to be discussed in detail later on in this application, it is believed that the program coupled with the advantages of using the CCD will render the instant system better and cheaper than any currently on the market or known to be under development.
The properties of the CCD that make it beneficial for incorporation into an X-ray system are the facts that CCDs are relatively low in cost and are seen to be extremely accurate in measuring the light falling upon them.
A further advantage to the use of the CCD in the X-ray system of this invention is the fact that the rapid cycling due to the presence of a shift register function allows the detected linear sequence of light measurements, which are in analog form based upon the number of electron pairs per element in the CCD, to be read out at high speed while the next measurement on the patient is being made. This ability contributes to the lack of necessity of the back and forth repetitive motion required in other types of X-ray apparatus tomographic systems.
Further, since the CCD is capable of operating at very low noise levels, especially when cooled, the signal that comes in can be detected even though it has not been amplified by any preceding stages. Whereas, however, in all other known prior art systems the input signal must undergo amplification. This benefit is extremely appealing to the X-ray practitioner since prior art computer tomographic apparatuses do not enjoy such a benefit.
In the device of this invention the only modification of the input signal that takes place is the focusing of same by a lens or fiber optic means. There is no amplification as that term is known by the CCD of the light signal. The CCD which is an electro-optical device creates electron pairs which can then be transmitted along a path on the chip to a pre-amplification stage. The outgoing electrical analog signal is converted into a digital signal for use with the computer program.
It is accordingly an object of the present invention to provide a method of and apparatus for practicing tomography which is substantially more efficient in time than other methods and is more accurate than prior art units.
A further object of the present invention is to provide a method of and an apparatus of the type discussed which permits one to obtain three dimensional information by radiation applied to a patient or other object in the order of two seconds or less, hence minimizing the likelihood of unsatisfactory results due to patient's movement, or breath holding inability.
Still another object of the present invention is to provide a system of the type referred to which utilizes an image sensor which inherently has greater accuracy than the devices utilized by the prior art.
Yet another object is to provide a means wherein the radiation is distributed more uniformly in that a 360.degree. revolution may be employed.
A still further object is to provide a tomographic device which employs an area image sensor or detector, rather than a single ray detector.
A yet further object is to provide a tomographic device which will give sufficient information for diagnostic purposes but with reduced exposure of the patient to radiation.
Other objects of the invention will in part be obvious and will in part appear hereinafter.